This invention relates to a manufacturing method of a frequency doubler (optical harmonic generating device) used in the field of optical information processing utilizing coherent light or in the field of measurement control applying light.
The structure of a conventional frequency doubler is shown in FIG. 6. Referring to this drawing, the optical harmonic generation (wavelength 0.42 .mu.m) to the fundamental wave of 0.84 .mu.m in wavelength using this frequency doubler is described in detail below. [See T. Taniuchi and K. Yamamoto, "Second harmonic generation by Cherenkov radiation in proton-exchanged LiNbO.sub.3 optical waveguide" CLEO '86, WR3, 1986.] When the light of fundamental wave P1 from a laser diode enters an incident plane 5 of a buried optical waveguide 2 formed on an LiNbO.sub.3 substrate 1, if the condition for equalizing the effective index N1 of the guided mode of the fundamental wave and the effective index N2 of the harmonic wave is satisfied, the light of the harmonic wave P2 is effectively emitted from the optical waveguide 2 into the LiNbO.sub.3 substrate so as to operate as an optical harmonic generating device.
Such conventional optical harmonic generating device required a buried type optical waveguide as a fundamental constituent element. The method of fabricating this buried type optical waveguide is explained by reference to FIG. 7. In FIG. 7A, a Ta film 13 is evaporated on the LiNbO.sub.3 substrate which is a ferroelectric crystal. In FIG. 7B, a slit of several micrometers in width is opened by photo process or etching using a photoresist 11. In FIG. 7C, heat treatment is effected in phosphoric acid to form a high refractive index layer (with the refractive index difference from the substrate of about .DELTA.Ne=0.14) as the waveguide 2.
The optical harmonic generating device fabricated in this manner, when the fundamental wave P1 with wavelength of 0.84 um enters one end of the waveguide 2 and the harmonic wave P2 is emitted from the other end, presents the maximum conversion efficiency at the thickness of 0.4 .mu.m of the waveguide with respect to the fundamental wave P1 of wavelength of 0.84 um from the laser diode, and a harmonic wave of P2=0.4 mW is obtained when the length of the waveguide is 6 mm and P1 is 40 mW. The conversion efficiency P1/P2 at this time is 1%. The propagation loss of this device is 1.5 dB/cm.
According to the research by the present inventors, in the optical harmonic generating device fabricated by using Ta mask as mentioned above, diffusion of Ta from the Ta mask to the LiNbO.sub.3 occurs, and the loss of the optical waveguide caused by this is about 1.5 dB/cm, which made it difficult to enhance the efficiency. For example, if the length of the waveguide is increased five times from 6 mm to 30 mm, the conversion efficiency of the device is not five-folded because of the propagation loss, and is actually about 2.7 times. When a short wavelength laser source was composed of the optical harmonic generating device using Ta mask, it was difficult to increase the output owing to the same reasons. It was therefore difficult to obtain stably harmonic waves of over 1 mW which is the practical level of the short wavelength laser source